The use of vibrating wire technology is known per se in the measurement instrument industry. The theory behind such technology is that a taut wire which has been made to vibrate resonates at a frequency which is a function of, among other things, the tension applied to the wire. If the wire is pre-tensioned, then a load applied generally axially with respect to the wire changes the tension on the wire and thus its resonating frequency. If the resulting frequency is measured, then the magnitude of load applied may be calculated theoretically therefrom or may be derived therefrom using known calibration techniques. As will be recognized by those skilled in the art, this measurement technique is readily adaptable for use in measuring strain or deformation.
Vibrating wire strain or deformation measurement technology has been found to be advantageous over conventional strain or deformation monitoring technology such as the use of metal foil gauges or mechanical extensometers. Metal foil strain gauge transducers are not recoverable once they are in place and they suffer from problems of creep due to thermal expansion and of drift during long term monitoring because the epoxy bond is subject to moisture absorption and shrinkage. In addition, the resistance of the read-out cable, which varies with length, directly affects signal output. With mechanical extensometers, increased accuracy tends to result in increased costs of manufacture, so it is difficult to attain a high level of accuracy without substantial expenditure. Additionally, extensometers typically require long base lengths to record deformation in the elastic range. These long base lengths tend to average out significant deformation gradients and thus their importance. Vibrating wire strain monitors on the other hand are relatively inexpensive to produce and they exhibit highly accurate results. For more information with respect to the principles of operation of vibrating wire strain monitors, reference may be made to applicant's U.S. Pat. No. 4,858,472 issued Aug. 22, 1989 or applicant's pending U.S. patent application Ser. No. 07/426,072 filed Oct. 23, 1989, now U.S. Pat. No. 5,048,344.
Heretofore, various strain monitoring/measuring devices have been proposed, among these are applicant's above-referenced patents. However, these devices have been directed primarily to the uni-directional measurement of strain or deformation. Since the deformation which occurs within rock typically does so in three dimensions, it would be useful to have a single integral device capable of precise strain or deformation monitoring also in three dimensions.